For example, for a steel sheet used for skeleton parts for automotives, high strength is required for the purpose of collision safety and fuel efficiency by forming lighter automotive as well as excellent processability for processing the skeleton parts having complicated shapes is also required.
Consequently, providing a high-strength steel sheet having tensile strength (TS) of 780 MPa class or more together with having improved balance between elongation (total elongation; El) and stretch flangeability (a hole expansion rate; λ) is earnestly required. For example, a steel sheet having tensile strength (TS) of 780 MPa or more, TS×El of 14000 MPa·% or more and TS×El×λ of 800000 MPa·%·% (more preferably TS of 780 MPa or more, TS×El of 15000 MPa·% or more and TS×El×λ of 1000000 MPa·%·% or more, and further preferably TS of 780 MPa or more, TS×El of 16000 MPa·% or more and TS×El×λ of 12000000 MPa·%·% or more) is required.
To accept the above-described requirements, a large number of high-strength steel sheets which have improved balance between elongation and stretch flangeability are suggested, based on various concepts for structure control. However, actual status is that only a few cases satisfy balance between the elongation and the stretch flangeability in the above-described required level.
For example, Patent Document 1 discloses a high-tension cold-rolled steel sheet including at least one of Mn, Cr and Mo of 1.6-2.5% by mass in total, substantially made of a single-phase structure of martensite. Although its hole expansion rate (stretch flangeability) λ of 100% or more is obtained in a steel sheet having tensile strength of 980 MPa class, its elongation El does not reach to 10%, and thereby the required level is not satisfied.
In Patent Document 2, a high-tension steel sheet made of two-phase structure which is made of ferrite of 65-85% in area ratio and remainder of tempering martensite is disclosed.
In Patent D 3, a high-tension steel sheet made of two-phase structure which has both of average crystal grain sizes of ferrite and martensite of 2 μM or less and includes martensite of 20% or more to less than 60% in a volume ratio is disclosed.
Any high-tension steel sheets disclosed in Patent Document 2 and Patent Document 3 ensure elongation exceeding 10% by mixing with large quantity of ferrite, which has high deformation ability, and some sheets satisfying the required level exist. Inventions according to these high-tension steel sheets is characterized in that an area proportion between ferrite and a hard phase, and grain sizes of these both phases are controlled. However, these inventions clearly differ from the present invention in technological idea which is characterized in that an amount of strain in ferrite, deformation ability of a hard phase, and further distribution state of cementite grains excising at an interface between the ferrite and the hard phase are controlled.